Ursa Major III
| Ursa Major III / UNIONS 1 | |
|---|---|
| Observation data (J2000 epoch) | |
| Constellation | Ursa Major |
| Right ascension | 11h 38m 49.8s[1] |
| Declination | +31° 04′ 42″[1] |
| Distance | 32.6±3.3 kly (10±1 kpc)[1] |
| Absolute magnitude (V) | +2.2+0.4 −0.3[1] |
| Characteristics | |
| Type | dSph[1] |
| Mass/Light ratio | 6500[1] M☉/L☉ |
| Number of stars | 57+21 −19[1] |
| Half-light radius (physical) | 4.8[2] |
| Half-light radius (apparent) | 0.9′[1] |
| Other designations | |
| UMa III, UNIONS 1 | |
Ursa Major III / UNIONS 1 (UMa III / U1) is a dwarf satellite galaxy of the Milky Way[1] or a stellar cluster.[2] If confirmed as a galaxy it would represent the smallest and faintest galaxy ever discovered, containing only about 60 star systems as opposed to hundreds of billions.[3] It was found by the deep, wide field Ultraviolet Near Infrared Optical Northern Survey (UNIONS), a collaboration between the Canada–France–Hawaii Telescope and Pan-STARRS (two observatories in Hawaii) and spectroscopically confirmed as a real satellite based on data obtained with the Keck Observatory's Deep Imaging Multi-Object Spectrograph (DEIMOS).
Ursa Major III's discovery was announced in November 2023, with a paper appearing in The Astrophysical Journal in January 2024. It contains a metal-poor stellar population, indicating an extreme age of 11 billion years. Located about 32,600 light years away, it has a diameter of just 19.6 light years and is thought to contain only about 60 stars. Combined with its absolute magnitude of only +2.2, this makes it by far the Milky Way's dimmest satellite, and only about as bright as Altair. This absolute magnitude corresponds to a total luminosity of 11.4 L☉.[1]
Dynamical measurements of Ursa Major III / UNIONS 1's stars suggest it may have a mass-to-light ratio of about 6,500. However, this becomes only 1,900 with the removal of one of the stars suspected to be part of the system. This very high value may indicate the presence of a massive dark matter halo, suggesting that Ursa Major III could indeed be a true dwarf galaxy, albeit one with an extremely low stellar mass of 16 M☉, making it the least massive Milky Way satellite known.[1]
A 2025 study found the previous estimate of the stellar mass to be highly underestimated, since it does not account for the large number of compact stars within the galaxy, which make 50 to 80% of Ursa Major III's mass and are invisible by UNIONS. Accounting for this the stellar mass would be 142 M☉. Simulations show UNIONS 1, without dark matter, can survive for 2.7 billion years with this higher mass. This is substantially larger than the 400 to 800 million years previously estimated, and as of such it can be a star cluster rather than a galaxy.[2]
References
[edit]- ^ a b c d e f g h i j k Smith, Simon E. T.; Cerny, William; Hayes, Christian R.; Sestito, Federico; Jensen, Jaclyn; McConnachie, Alan W.; Geha, Marla; Navarro, Julio; Li, Ting S. (January 2024). "The discovery of the faintest known Milky Way satellite using UNIONS". The Astrophysical Journal. 961 (1): 92. arXiv:2311.10147. Bibcode:2024ApJ...961...92S. doi:10.3847/1538-4357/ad0d9f.
- ^ a b c Devlin, Scot; Baumgardt, Holger; Sweet, Sarah M (2025-05-21). "Reevaluating UMa3/U1: star cluster or the smallest known galaxy?". Monthly Notices of the Royal Astronomical Society. 539 (3): 2485–2497. doi:10.1093/mnras/staf572. ISSN 0035-8711.
- ^ Kuthunur, Sharmila (3 April 2024). "Group of 60 faint stars orbiting the Milky Way could be new type of galaxy never seen before". Live Science. Retrieved 10 April 2024.
| Location | Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of". |  | |||||
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